1. Field of the Invention
The present invention relates to a semiconductor device and particularly relates to a semiconductor device having a ferroelectric capacitor.
Semiconductor memory devices such as DRAMs and SRAMs are widely used as high-speed main memory devices for information processing devices such as computers. However, such semiconductor memory devices are volatile in nature and thus information stored thereon is lost when the power supply is turned off. Non-volatile magnetic disk devices are commonly used as large-scale auxiliary storage devices for storing programs and data.
However, magnetic disk devices are bulky, vulnerable to mechanical shocks and have a large consumption power. A further drawback of magnetic devices is a slow access speed during information read/write operations. In order to obviate such drawbacks, recently, EEPROMs or flash memories that store information by accumulating electric charges on floating gate electrodes are often used as non-volatile auxiliary storage devices. Particularly, flash memories have a cell structure similar to DRAMs and can be formed at a large integration density. Therefore, flash memories are becoming of an interest as large-scale memory devices that are comparable to magnetic disk devices.
In an EEPROM or a flash memory, information is written by injecting hot electrons into floating gate electrodes via tunnel insulation film. Such memory devices have drawbacks that a writing operation is time consuming and that the tunnel insulation films deteriorate due to repeatedly performed information write/erase operations.
In order to obviate such drawbacks, a ferroelectric memory device (hereinafter referred to as FeRAM) has been proposed that stores information in the form of spontaneous polarization. Such FeRAM has a structure similar to that of the DRAM in that each memory cell transistor is provided as a single MOSFET. Further, a dielectric film in the memory cell capacitor is replaced by a ferroelectric material such as PZT (Pb(Zr,Ti)O3) or PLZT ((Pb,La)(Zr,Ti)O3) and further by SBT(SrBi2Ta2O3). The FeRAM is capable of being integrated at a high integration density.
The ferroelectric semiconductor memory device controls the spontaneous polarization of the ferroelectric capacitor by applying electric fields, so that a writing speed becomes faster by a factor of 1000 or more as compared to an EEPROM or a flash-memory in which information is written by injecting hot electrons into the floating gate through the tunneling insulation film. Further, the FeRAM is advantageous in that the power consumption is reduced to about 1/10 that of an EEPROM or a flash-memory. Further, since a tunneling insulation film is not required, the FeRAM has an increased lifetime and can perform writing operations of about one hundred thousand times that of a flash-memory device.
2. Description of the Related Art
FIG. 1 shows the construction of a conventional FeRAM 10.
As shown in FIG. 1, the FeRAM 10 is constructed on a p-type Si-substrate 11 and is formed on a p-type well 11A having an active region defined by a field oxide film 12. In the active region, a gate electrode 13 is formed in correspondence to a word line of the FeRAM via a gate oxide film that is not shown in the figure. Further, n+-type diffusion regions 11B and 11C are formed in the substrate 11 on both sides of the gate electrode 13 to serve as a source region and a drain region, respectively, of the memory cell transistor. A channel region is formed in the p-type well 11A at a position between the diffusion regions 11B and 11C.
The gate electrode 13 is covered by a CVD oxide film 14 that covers the surface of the Si substrate 11 in correspondence to the active regions. A ferroelectric capacitor C including a lower electrode 15, a ferroelectric film 16 such as a PZT film formed on the lower electrode 16 and an upper electrode 17 formed on the ferroelectric film 16 is formed on the CVD oxide film 14. The ferroelectric capacitor C is covered with an insulation film 18 such as a CVD oxide film. The upper electrode 17 is electrically connected to the diffusion region 11B via a local interconnection pattern 19A that contacts the upper electrode 17 at a contact hole 18A formed in the insulation film 18 and contacts the diffusion region 11B at a contact hole 18B formed in the insulation films 18 and 14.
Further, via a contact hole 18C formed in the insulation films 18 and 14, the diffusion region 11C is electrically connected to an electrode 19B that forms a bit-line of the FeRAM 10. The entire surface of the thus-formed FeRAM 10 is protected by a protection insulation film 20.
In a conventional ferroelectric capacitor, the lower electrode 15 is often made of a Ti/Pt stacked film and the ferroelectric film 16 provided on the lower electrode 15 is made of a PZT film. For such a ferroelectric capacitor, the Pt film forming the lower electrode 15 is mainly formed of Pt polycrystals oriented in the <111> direction. Therefore the orientation of the ferroelectric film 16 formed on the Ti/Pt stacked film is dominated by the orientation of the lower electrode and, as a result, is mainly oriented in the <111> direction. That is to say, it is known that such a ferroelectric capacitor has a so-called (111) orientation (see J. Appl. Phys, vol. 70, No. 1, 1991, pp. 382-388).
FIG. 2 is a schematic diagram showing a structure of a ferroelectric capacitor C of the related art.
Referring to FIG. 2, the ferroelectric capacitor insulation film 16 has a micro-structure of columnar PZT crystals extending from the lower electrode 15 to the upper electrode 17 with each columnar PZT crystals being oriented in the <111> direction. It is known that the PZT crystal belongs to a tetragonal system and has spontaneous polarization in the <001> direction. In such columnar crystals oriented in the <111> direction, the direction of polarization will be inclined against a direction of electric field connecting the upper and lower electrodes 15 and 17, as shown by arrows in FIG. 2.
FIG. 3 is a graph showing electric characteristics of such a ferroelectric capacitor. In FIG. 3, the vertical axis represents polarization and the horizontal axis represents an applied voltage. In FIG. 3, white circles indicate electric characteristics for ferroelectric capacitor of FIG. 2, when PZT crystals are oriented in the <111> direction. Black circles indicate, electric characteristics for the ferroelectric capacitor of FIG. 2, when PZT crystals are oriented in the <001> direction.
As can be seen in FIG. 3, the ferroelectric capacitor clearly presents, for either case, hysteresis properties that are specific to ferroelectric materials. As is readily understood, the ferroelectric capacitor has a greater remanent polarization and a better retention property when the PZT crystals in the capacitor insulation film 16 are oriented in a direction of applied electric field, or the <001> direction, as compared to a case where they are oriented in a direction inclined against the applied electric field, or the <111> direction.
When such a ferroelectric capacitor is used as the capacitor C for the FeRAM shown in FIG. 1, information can be retained in the form of remanent polarization of the capacitor C. The state of polarization of such a ferroelectric capacitor can be read out at the bit-line 19B via a transistor having the diffusion regions 11B, 11C and the gate electrode 13. Also, during a writing operation or an erasing operation, a predetermined writing voltage is applied to the bit-line 19B to turn on the transistor so as to apply a voltage between the electrodes 15 and 17 of the ferroelectric capacitor C, that is sufficient for reversing the polarization property of FIG. 3.
With such a ferroelectric capacitor, as shown in FIG. 4, a phenomenon called fatigue or deterioration of retention property occurs in which the value of remanent polarization Pr, that is to say, the retention property decreases with time. Also, it is known that a phenomenon called an imprint deficiency occurs when “1” or “0” is repeatedly written. Such imprint deficiency can be seen from FIG. 5, in which it is shown that the coercive voltage Vc of FIG. 3 shifts with time.
FIG. 6 is a diagram showing a retention property of a ferroelectric capacitor using a ferroelectric film of PZT polycrystals oriented in the <111> direction in comparison to a retention property of a ferroelectric capacitor using a ferroelectric film of PZT polycrystals oriented in the <001> direction.
Referring to FIG. 6, it can be seen that a fatigue clearly occurs for the ferroelectric capacitor using the ferroelectric film of PZT polycrystals oriented in the <111> direction. On the contrary, it can be seen that hardly any fatigue occurs for the ferroelectric capacitor using the ferroelectric film of PZT polycrystals oriented in the <001> direction. It may be understood that for the ferroelectric film of PZT polycrystals oriented in the <111> direction, when directions of polarization are different between a pair of neighboring domains, strain is accumulated in a domain wall, and a defect due to the strain causes deterioration of the retention property of the ferroelectric film. For the ferroelectric capacitor using the ferroelectric film of PZT polycrystals oriented in the <001> direction, directions of polarization are parallel between neighboring domains and therefore no such accumulation of strain occurs in the domain wall.
FIG. 7 is a graph showing an amount of coercive voltage shift for the same ferroelectric capacitor.
Referring to FIG. 7, it can be seen that the coercive voltage shift occurs for the case where the PZT film is oriented in the <001> direction in a similar manner to the case where the PZT film is oriented in the <111> direction. Based on a recognition that the deterioration with time of the polarization hardly occurs for the PZT film oriented in the <001> direction shown in FIG. 6, it can be assumed that any shift of the coercive voltage Vc occurring in the PZT film is not due to the strain in the domain wall shown in FIG. 2 or deterioration of the PZT film itself. Such deterioration of an imprint property can be assumed as being due to electric charges accumulated near a boundary surface between the PZT film 16 and the upper electrode 17 or the lower electrode 15 adjacent thereto.